Injection molding is the plastic raw material after granulation, dying, adding additives, and other processing of granular materials) into the cylinder, after heating plasticizing, making it the high viscosity of the fluid (melt), pressurized with piston or screw as a tool, make the melt with high pressure (about 25 ~ 80 Mpa) through the nozzle injection mold cavity, after cooling, solidified and demolding, become plastic products. Defects caused by plastic forming may affect the assembly efficiency or the performance of the whole machine. These defects may be caused by mold, raw materials, process parameters, equipment, environment, personnel, and other reasons. Here we summarize the defects as follows:

Moisture Streaks

Some infusible layers of material are created on the surface of the mold to separate the parts (possibly water, gas) or to prevent the plastic from completely copying the original mold surface when it enters the mold.

Glass Fiber Streaks

Based on the length of glass fiber, or the relationship between the direction before melting flow and the direction, the change of flow speed and cooling speed, the bad arrangement of glass iron, and the appearance of float surface of glass domain (uneven distribution of glass fiber) due to vertical and oblique reasons, this situation will not only affect the appearance but also reduce the engineering structure of injection parts.

Gas Trap Effect

This condition is caused by the presence of poor air emissions in the mold, which can be caused by design, material, or poor injection molding conditions.

Silver Marks

This situation is caused by the fact that the hot-melt plastic in the mold or the injection cylinder has a large amount of gasified water or the gas/water caused by the chemical reaction of the additive, which is injected into the mold cavity and pressed to radiate to form the silvery ridges during the injection molding process.

Weld Line

In the process of injection molding parts in the inflow of bad clamping produced bonding line, in the process of flow will also produce a similar fusion line. Both of them basically have the same original trap. Whether the plastic parts are poorly designed or the injection molding conditions are not good enough, the most important reason is that the confluence flow peak produces a temperature of = or >10 degrees.

Water Wave Effect

Injection parts in the process of the inflow of bad cooling conditions, injection speed, or melt temperature are too low, which will produce water ripples. Or the plastic side into the side of rapid cooling, that is not the smooth movement of the plastic pressure to the wall and the plastic into the form of radiation propulsion after cooling the surface so that the formation of like water ripples/finger pattern.

Dark Spots

The black spots on the injection parts are all made of impurity. This person has many problems such as injection molding operation control, mold cleaning and wear, injection molding machine barrel cleaning and wear, raw material pollution/turn over abrasive pollution.

Visible Ejector Marks

Visible Ejector Mark is caused by ejection difficulties. And the demolding difficulty is basically due to the plastic parts in the mold being pushed to cure insufficient and mold design/poor production.

Deformation During Demolding

The deformation of the injection parts does not occur until/after demolding. The first is deformation due to poor curing conditions/poor die design, and the second is deformation due to poor injection pressure conditions provided during the injection molding process.

Short FiIling

Insufficient filling occurs in the following environments, one is a poor injection molding operation, the other is a poor injection molding part design. Most defects caused by injection processing factors are caused by an excessive temperature drop and pressure drop during the injection process from the gate to the completion of filling. Defects are caused by poor design of injection parts, most of which are caused by gate opening too far away, uneven thickness, overall thin glue level, special design restrictions, poor exhaust, etc.

Cold Slug

Cold slug material flow marks are caused when molten plastic passes through a gate or nozzle, solidifies slightly in advance, and enters the cavity along with the molten plastic in the rear section. The situation is as serious as the non-profound combination line.

Dull Spots Near The Spurs

This defect is actually high shear stress cracking, mostly in the submerged gate and plastic parts with high viscosity. When the high viscosity molten plastic passes through the very thin opening, it needs high speed and high pressure to pass through so that the overloading cracking defect will occur near the entrance gate.


Injection jetting mark is the plastic injection molding machine plasticization of the amount of insufficient, or the plastic in the state of incomplete melting through the gate and after the molten plastic into the cavity when the phenomenon.

Flaking Of The Surface Layer

It is the plastic formulation of the injection molding part that produces incomplete fusion after forming or the plastic formulation (including additives) that produces repulsive splitting after forming.

Entrapped Air

This is the injection part in the melting process or in the use of excessive back wire function mixed with a lot of air, and in the injection process into the mold cavity. The plastic material is transparent grade and is more easily found.

Burnt Streaks

There are several reasons for this defect. Thermal discoloration occurs when a polymer is heated too much and the chains of molecules break apart and are either thermally degraded or burned. Sometimes it’s just a yellowish discoloration or slight thermal degradation, which is not only caused by the burning of the polymer, but also by the presence of the rotator inside the polymer. If the melt does not appear as a brown, yellow, or black color change in the cleaning machine, it indicates that the melt begins to burn after leaving the nozzle. The main diameter, flow path, water level, and mold design should be checked or injection speed and pressure should be corrected. If coffee streaks or discoloration are found in the cleaning machine, it proves that degradation occurs in the barrel and nozzle. If a stripe of coffee and argent stripe occurs at the same time, that is a moisture element certainly.

Stress Crack

This defect is caused by the excessive shear stress of the plastic in the process of wang molding. Referring to the plastic under the condition of insufficient heating, flow is not good, forced to push it with greater pressure into the cavity, at this time the molecular excessive elongation aging, so that it is easy to fracture and turtle crack, transparent plastic more easily see this phenomenon.

Sink Marks

There are many reasons for this phenomenon, such as improper selection of plastic characteristics, poor plastic part design, and poor injection conditions. The main performance will be slow plastic curing, insufficient effective pressure holding time, or poor pressure holding transmission.

Density Bubble

The defect is mainly caused by the insufficient density of the plastic in the injection molding process. The failure to fill the plastic parts with adequate density is related to the type of plastic (crystalline/amorphous), premature stop of the flow rate into the plastic, or poor plastic parts design.


This defect is mainly caused by excessive plastic filling during the injection molding process. Plastic parts excessive filling and plastic grade (flow index MFI), plastic parts bad design, mold damage or injection molding machine type, injection molding machine efficiency and adjustment.

Scratch Marks

This phenomenon is mainly caused by excessive friction between the injection parts and the wall surface and the corner of the mold during the demolding process after the injection. It is mainly related to the insufficient demolding slope of plastic parts, the insufficient structural strength of labor mold, damage of labor mold, or the efficiency of injection molding machine and adjustment of injection molding parameters.